1. Field of the Invention
This invention relates generally to pulse-width modulated, current regulated utility interface converters. More particularly, the present invention relates to control of the pulse-width modulated converter to provide a utility current wave form with low harmonic content, variable power factor, and efficient energy conversion.
2. Description of Related Art
Utility interface power electronic converters are commonly utilized to exchange electrical energy between an AC source, such as a utility, and DC source, or DC load. Conventional power electronic converters use power switching devices such as a silicon controlled rectifier in a line commutated converter topology. In recent years, the conversion function has been accomplished by the use of more advanced devices, such as power transistors, gate-turn-off thyristors, metal-oxide-field-effect-transistors, or other gate controlled devices in a self commutated, pulse-width modulated converter topology. These later converters represent an improvement in cost as they inherently can pass electrical energy bi-directionally, i.e. from AC to DC or from DC to AC, and can eliminate the high magnitude, low frequency harmonic currents associated with line commutated converters. However, one drawback of the pulse-width-modulated converter is the generation of high frequency harmonic currents which are at, or near, the converter switching frequency. These harmonic currents interact with utility system impedances and distort the utility voltage leading to potential disturbance problems for other sensitive equipment which may be connected to the utility. Because of this and related problems, the need to reduce or eliminate high frequency harmonic currents is well recognized.
Traditional approaches to mitigating harmonic current problems include installation of passive element power line filters. Although such passive filters can reduce harmonic content, they have undesirable side effects associated with them, most notably the addition of pure capacitance on the line which can interact with normal utility system distributed inductances resulting in resonant modes. If any equipment on the utility excites the inductive-capacitive network at its resonant frequency, then utility system instability can result. To eliminate this possibility, it is necessary to add resistance to the capacitive network to provide damping at the resonant frequency. However, the addition of resistance to the network has the negative side effect of considerable losses, the extent of which depends on converter size but can easily be in the two-to-three kilowatt (or larger) range. Also, because the filter principally has a capacitive reactive nature, voltage harmonics present in the utility, such as harmonics produced as a by-product of power generation, result in additional filter induced utility harmonic currents at the voltage harmonic frequencies.
In more recent years, active power filters have been proposed which electronically compensate for low frequency harmonics, such as 5.sup.th, 7.sup.th, 11.sup.th and 13.sup.th harmonics generated by line-commutated converters. One such active filter includes a self-commutated pulse-width-modulated converter coupled in parallel with the line-commutated converter. These active filters are incapable of filtering the high frequency components generated by self-commutated, pulse-width-modulated converters, and in fact, since these are self-commutated converters in and of themselves, they introduce high frequency components by their application.
It would be advantageous to provide a power filter that more effectively reduces the high frequency harmonic currents generated by self-commutated converters, thereby providing a more acceptable power converter configuration for connection to the utility. It would also be an advantage to provide a power filter which can substantially reduce, or eliminate, the need for the large damping resistor, thereby reducing the losses in the filter. Such a power line filter would provide low harmonic current content to the utility and would have high efficiency for effective electrical energy transfer.